This is a "feature" of ubiquity (the ubuntu installer), and very disappointing indeed. Most other distros ask you where you want to install grub, if not the mbr. Some are smarter, and just ask if you want to install grub to mbr or root, making it very simple. I've tried ubiquity twice now, once to install Gutsy and once to install Mint. From what I remember, the little box where you could have placed an alternative to mbr had a header called Grub Help, or something like that. Very odd. Even the old text based Debian installer was easier to understand in this regard. But then again, Mint is just inheriting this oddity from Ubuntu, I guess. This is the only thing I haven't liked so far with this distro, which is only a week old on my home Linux box. Otherwise, it's becoming one of my favorite distros ever.